The regulation of the cell cycle is governed and controlled by specific proteins, which are activated and deactivated mainly through phosphorylation/dephosphorylation processes in a precisely timed manner. The key proteins that coordinate the initiation, progression, and completion of cell-cycle program are cyclin dependent kinases (CDKs). Cyclin-dependent kinases belong to the serine-threonine protein kinase family. They are heterodimeric complexes composed of a catalytic kinase subunit and a regulatory cyclin subunit. CDK activity is controlled by association with their corresponding regulatory subunits (cyclins) and CDK inhibitor proteins (Cip & Kip proteins, INK4s), by their phosphorylation state, and by ubiquitin-mediated proteolytic degradation (see D. G. Johnson, C. L. Walker, Annu. Rev. Pharmacol. Toxicol 39 (1999) 295-312; D. O. Morgan, Annu. Rev. Cell Dev. Biol. 13 (1997) 261-291; C. J. Sherr, Science 274 (1996) 1672-1677; T. Shimamura et al., Bioorg. Med. Chem. Lett. 16 (2006) 3751-3754).
There are four CDKs that are significantly involved in cellular proliferation: CDK1, which predominantly regulates the transition from G2 to M phase, and CDK2, CDK4, and CDK6, which regulate the transition from G1 to S phase (Malumbres M, Barbacid M. Cell cycle, CDKs and cancer: a changing paradigm. Nat. Rev. Cancer 2009; 9(3):153-166). In early to mid G1 phase, when the cell is responsive to mitogenic stimuli, activation of CDK4-cyclin D and CDK6-cyclin D induces phosphorylation of the retinoblastoma protein (pRb). Phosphorylation of pRb releases the transcription factor E2F, which enters the nucleus to activate transcription of other cyclins which promote further progression of the cell cycle (see J. A. Diehl, Cancer Biol. Ther. 1 (2002) 226-231; C. J. Sherr, Cell 73 (1993) 1059-1065). CDK4 and CDK6 are closely related proteins with basically indistinguishable biochemical properties (see M. Malumbres, M. Barbacid, Trends Biochem. Sci. 30 (2005) 630-641).
A number of CDK 4/6 inhibitors have been identified, including specific pyrido[2,3-d]pyrimidines, 2-anilinopyrimidines, diaryl ureas, benzoyl-2,4-diaminothiazoles, indolo[6,7-a]pyrrolo[3,4-c]carbazoles, and oxindoles (see P. S. Sharma, R. Sharma, R. Tyagi, Curr. Cancer Drug Targets 8 (2008) 53-75). For example, WO 03/062236 identifies a series of 2-(pyridin-2-ylamino-pyrido[2,3]pyrimidin-7-ones for the treatment of Rb positive cancers that show selectivity for CDK4/6, including 6-acetyl-8-cyclopentyl-5-methyl-2-(5-piperazin-1-yl-pyridin-2-ylamino)-8H-pyrido-[2,3-d]-pyrimidin-7-one (PD0332991), which is currently being tested by Pfizer in late stage clinical trials as an anti-neoplastic agent against estrogen-positive, HER2-negative breast cancer. VanderWel et al. describe an iodine-containing pyrido[2,3-d]pyrimidine-7-one (CKIA) as a potent and selective CDK4 inhibitor (see VanderWel et al., J. Med. Chem. 48 (2005) 2371-2387). WO 99/15500 filed by Glaxo Group Ltd discloses protein kinase and serine/threonine kinase inhibitors. WO 2010/020675 filed by Novartis AG describes pyrrolopyrimidine compounds as CDK inhibitors. WO 2011/101409 also filed by Novartis describes pyrrolopyrimidines with CDK 4/6 inhibitory activity. WO 2005/052147 filed by Novartis and WO 2006/074985 filed by Janssen Pharma disclose additional CDK4 inhibitors. WO 2012/061156 filed by Tavares and assigned to G1 Therapeutics describes CDK inhibitors. WO 2013/148748 filed by Francis Tavares and assigned to G1 Therapeutics describes Lactam Kinase Inhibitors.
While selective CDK4/6 inhibitors are generally designed to target CDK4/6-replication dependent cancers, the very fact that they inhibit CDK4/6 activity may also result in deleterious effects to CDK4/6-dependent healthy cells, for example their growth inhibition. CDK4/6 activity is necessary for the production of healthy blood cells by the bone marrow, as healthy hematopoietic stem and progenitor cells (HSPCs) require the activity of CDK4/6 for proliferation (see Roberts et al. Multiple Roles of Cyclin-Dependent Kinase 4/6 Inhibitors in Cancer Therapy. JNCI 2012; 104(6):476-487). Healthy hematopoietic stem cells give rise to progenitor cells which in turn give rise to all the differentiated components of blood as shown in FIG. 1 (e.g., lymphocytes, erythrocytes, platelets, granulocytes, monocytes). Healthy hematopoietic cells display a gradient dependency on CDK4/6 activity for proliferation during myeloid/erythroid differentiation (see Johnson et al. Mitigation of hematological radiation toxicity in mice through pharmacological quiescence induced by CDK4/6 inhibition. J Clin. Invest. 2010; 120(7): 2528-2536). Accordingly, the least differentiated cells (e.g., healthy hematopoietic stem cells (HSCs), multi-potent progenitors (MPPs), and common myeloid progenitors (CMP)) appear to be the most dependent on CDK4/6 activity for proliferation, and therefore the most deleteriously affected by the use of a CDK4/6 inhibitor to treat a CDK4/6 replication dependent cancer or other proliferative disorder.
Accordingly, there is an ongoing need for improved compounds, methods, and regimes to treat patients with select Rb-positive cancers and abnormal cellular proliferative disorders while minimizing the treatment's effect on healthy cells such as HSPCs.